After changing login url through security plugin , login page layout design is chnaged, not looking like as your demo polytechnic theme login page
Thanks for the question. If you've changed the native WordPress login url, then it's likely related to that.
I'd recommend reaching out to the plugin author, since this falls beyond the scope of theme support (ie. Out of the Box theme functionality/support).
Standard Troubleshooting Steps, Support Policy
To provide some context - the theme provided custom login simply loads a "login-styles.php" file in functions.php, which enqueues styles and scripts based on (1) the css/js included in polytechnic/theme-core/theme-functions/login files and (2) the option values in Appearance > Theme Options > Custom Login.
If you are having issues with the security plugin, you turn Custom Login OFF in Appearance > Theme Options > Custom Login. Beyond that, you would need to identify what is loading/not-loading and work through this custom security plugin integration with the plugin author to customize the login page to your requirements, since the plugin is leveraging any number of functions/files/scripts/styles to override/extend this native WordPress functionality.